This proposal contains plans for studies on several different topics in transfer RNA research: (i) genetic and biochemical studies on the regulation of the biosynthesis of tRNA and aminoacyl-tRNA synthetases; (ii) comparative studies on the structure of specific tRNA precursors from E. coli, yeast and mammalian cells, (iii) the preparation and use of tRNAs with "base substitutions" or modified with fluorescent or photoreactive groups; (iv) biochemical analysis of nonsense suppressor tRNAs in yeast; and (v) biochemical analysis of E. coli "mischarging" mutants which have tRNAs with altered amino acid specificity.